What is a good and inexpensive technique for taking pictures of flat items like napkins for a products page?
two light sources at 45 degrees to the surface - camera mounted at 90 degrees to the surface of the object
If you have done it that way with good results I would love to see some photos. For the OP, I agree with the lights at 45 degrees at a setting of 4PM and 8PM with the camera at 6PM and also at 45-60 degrees. The PM were just added to indicate clocks hand, you can use AM if you like.
product photography is something that needs time, effort and some $. The best way for small objects is to make yourself a lightbox or light tent. You will need a good quality camera, a good tripod and at least 2 light sources (and up to 5 light sources), a couple of different backgrounds (ie a white one, and a non-white one). The other choice is to pay someone to do it for you.
Did you use napkins just as a hypothetical example? Comic book pages etc would work well with McFox's description above, but if you do actually want to sell napkins, folded 3d structures would possibly be better.
Fix (with adhesive) all product napkins on to the nice colored background wooden board and take a snap from suitable distance. Slice them in photoshop as per your product page requirement.